This invention relates generally to techniques of monitoring a process being performed to transform a surface of an article, an example being a semiconductor wafer, such as by monitoring the formation or removal of a layer of material. A specific application of the invention is to monitor the progress of a chemical-mechanical-polishing ("CMP") process on a surface carried by a semiconductor wafer, including detecting an endpoint of the process.
CMP, usually resulting in planarization of a surface, is a key technology used for the fabrication of advanced microelectronic devices (e.g. those having critical dimensions that are less than about 0.5 micron) on substrates. Planarization allows smaller features to be fabricated, which leads to denser circuit packing and faster circuit response, which in turn results in faster logic devices and higher capacity memories. In addition, planarized surfaces allow for the fabrication of circuit devices having multiple levels of metallization, and therefore a more sophisticated design. The process consists of mechanically grinding or polishing of the work piece (generally a thin film microelectronic circuit imposed on a silicon wafer) against a rotating polishing wheel (platen), in the presence of a chemical slurry therebetween. The slurry provides both abrasive particles (to accomplish the mechanical grinding) and reactive chemicals (to assist the film removal by chemical modification of the film surface). The wafer is typically held (by frictional forces) to a polishing head or wafer carrier, which itself rotates, and which provides downward pressure of the wafer against the platen. A porous polymeric polishing pad mounted on the platen helps to trap the abrasive slurry particles, and provides a conformable surface that aids in the planarization process.
Process control, and specifically endpoint control, has become a key issue in CMP, in part because of process variations, such as in the above mentioned slurry and polishing pad, and in part because of variations in the thickness, composition, and uniformity of the films to be polished. U.S. Pat. No. 5,499,733 is directed to such process control. This patent is hereby incorporated herein by this reference. Briefly, an interrogating optical beam having a wavelength to which the wafer is substantially transparent is directed through the back side of the wafer being polished. The state of removal of the film on the front side of the wafer is then inferred from the time evolution of the amount of the interrogating light which is reflected from the film surface. In the case of opaque (e.g. metal) films, a pronounced shift in reflected light intensity often occurs when the film has been completely removed, thus signaling an endpoint of the process. In the case of transparent films (e.g. silicon dioxide and other dielectrics), the time evolution generally consists of alternating intensity peaks and valleys caused by the well known effects of thin film optical interference. By analyzing the pattern of peaks and valleys, one can determine the amount of film that has been removed, thus allowing an endpoint of the process to be called when a predetermined amount of film removal has occurred.
Pending U.S. patent application Ser. No. 08/585,164, filed Jan. 11, 1996, describes a specific CMP machine. This application is incorporated herein by this reference. In that machine, multiple optical sensors are distributed across the wafer carrier in order to monitor the uniformity of material removal across the wafer. These sensors are fixed to the rotating wafer carrier and communicate with a stationary processor by a wireless link. An end point to the process is determined after a certain percentage of the sensors have detected that an end point has been reached at their respective locations across the wafer. The amount of material being removed from a transparent layer is monitored in situ by counting the peaks, valleys and zero crossings of an optical interference signal obtained by each sensor. The interference signal may also be subjected to a known curve fitting algorithm, such as the least squares algorithm, to improve the accuracy of the measurement.
These same transparent layer thickness monitoring techniques can be applied to the growth or deposition of a transparent film onto a substrate such as a semiconductor wafer. A popular technique for depositing a transparent film of material, such as silicon dioxide, onto a substrate is chemical vapor deposition ("CVD"). The increasing thickness of the film is optically monitored by the interference technique and an end point is detected when a desired thickness is obtained.
It is a primary object of the present invention to provide techniques for monitoring amounts of material being added to or removed from a transparent layer that provide initial measurements very early after the beginning of the process.
It is also an object of the present invention to provide improved techniques for detecting the break through of a transparent layer as a result of material removal.
It is another object of the present invention to provide techniques for monitoring and controlling CMP processes used in the manufacturing of integrated circuits, flat panel displays, and the like.